Advanced ultrasound activated lockin-thermography for defect selective depth-resolved imaging

Abstract

Ultrasound activated Lockin-Thermography ("ultrasound attenuation mapping") is a defect selective NDT-technique. Its main advantage is a high probability of defect detection ("POD") since only defects produce a signal while all other features are suppressed. The mechanism involved is local sound absorption which turns a variably loaded defect into a heat source. Thermographic monitoring of elastic wave attenuation in defects was reported for the first time in 1979 by Henneke and colleagues for continuous and pulsed ultrasound injection. Later, amplitude modulated ultrasound was used to derive frequency coded phase angle images combining defect-selectivity with robustness of measurement. With mono-frequent ultrasound excitation a standing wave pattern might hide defects. With additional modulation of the ultrasound frequency such a misleading pattern can be minimized. Applications related to quality maintenance (aerospace, automotive industry) will be presented in order to illustrate the potential of frequency modulated ultrasound excitation and its applications.